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Quantum entanglement question

  1. Jan 1, 2008 #1
    Hello:

    Sorry for asking this beginner question:

    Suppose we have two electrons that are entangled. Now, from what I understand, they are sharing the same wavefunction. Everywhere I read that information cannot travel faster than light in this case because the observer cannot change the state (he/she can only measure the state, and not influence it).

    But what if we combine it with an anti-electron and the whole thing converts to a photon. What will happen to the entangled electron on the other side of the universe? Suppose we have a large bunch of these entangled pairs; can we not pass information by destroying some of these and not destroying others (thereby forming a bit sequence)?

    Thanks in advance.
     
  2. jcsd
  3. Jan 2, 2008 #2

    DrChinese

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    You cannot pass a bit of information with an single entangled particle. Each particle still exhibits random behavior (such as spin being up or down). The entanglement does lead to well defined correlations, but these correlations don't become clear until the paired results are brought together. That doesn't allow for the possibility of Faster-Than-Light signalling.
     
  4. Jan 2, 2008 #3

    JesseM

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    Nothing at all, entanglement does not mean they continue to mirror each other in future interactions.
     
  5. Jan 2, 2008 #4
    As DrChinese said, the 'absolute' state remains random, however the state relative to the state of the entangled particle will have a relationship (either a definite one or probabilistic) depending, instantly, for example on the relative measurement angles. That means, there is an effect, since the state relative to the entangled particle is influenced in a way that cannot be explained by only the local particle being affected. However, the state of the local particle is not under control, so it cannot be used to transmit an arbitrary message. (And an attempt to control the local state may cause them to become disentangled.)

    What happens to the entangled photon when something is done to the other, depends on the action. For example a measurement of the entangled state will 'disentangle' this state, besides causing a correlation of the corresponding measurement. However, for example, entangling one of the particles, per Bell measurement, with a second pair of particles, will cause the entangled particle, however distant, to become entangled as well, with particles of that second pair. Also 'quantum teleportation' can be performed across the entangled 'connection', however requires an additional classical communication.
     
  6. Jan 2, 2008 #5
    colorSpace - could not entanglement of three or more particles be used to send FTL information? If particles A, B, and C are created in an entangled state, while A and B are sent to Alice and C is sent to Bob light years away, can Alice not detect whether Bob has measured C by examining the correlation probability of A and B, i.e. observing whether A and B remain entangled?
     
  7. Jan 3, 2008 #6

    DrChinese

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    That's a great question. Suppose that 3 particles are entangled:

    HHH + VVV

    It turns out that unlike the 2 photon entangled state (HH + VV), the 3 photon state does not simplify back to a "pure" expression as it is rotated through various angles. The 2 photon state can be rotated through angles (the math is a little messy) and the terms always cancel and you end up with HH + VV (2 terms). But with 3 particles, there ends up being 2 additional terms (total of 4). As a result, you can't expect A and B to yield the same measurement values except at the true horizontal or vertical (i.e. not rotated). At those settings, A & B always yield the same value regardless of what happens at C. At any other settings, A & B don't necessarily yield the same results - also regardless of C.
     
  8. Jan 3, 2008 #7
    Peter - an interesting question, it got me thinking. First one thing: As I see it, based on how several leading physicists describe the situation, in entanglement there is FTL information being sent - however it is, so to speak, XOR'ed with a random number and therefore not readable by looking at only one side, without additional classical communication. In quantum teleportation, quantum encryption and quantum computers, this is being used and technical applications are being developed based on it, however they require additional classical communication.

    I'm not sure about the details of the GHZ scenario regarding your question, I wish there was some kind of software simulation module which one could use in a building-block fashion to configure different scenarios and play with them. However such a simulation system would have to be quite sophisticated to be useful for more than one purpose.

    There is an "advanced" technique that might come even closer to what you are trying to do: 'entanglement swapping' , which I understand only superficially. Yet here some thoughts based on what I understand so far:

    For your purposes, two pairs of entangled pairs would be produced (A1,B1) and (A2,B2). Then, A can use a Bell measurement on A1 and A2, which if successful, will also entangle B1 and B2 (which is called entanglement swapping). Unfortunately, there are limiting factors, and the question is whether they could be overcome (I guess it is unlikely since clever people have already tried all kinds of things). Yet to a certain extent, B can then test whether B1 and B2 are entangled by seeing whether they are correlated. Of course, they could be correlated by coincidence, so A and B would have to use many double-pairs so as to get a statistical result. But one of the limiting factors is that they can be entangled in different ways, and so these different ways will again interfere with getting a clear result by looking at only B's side, I'm afraid. However, such scenarios make it more and more intuitive, that actions at A do effect the states at B, I think.

    Yet your idea is to start with a state that is known to be entangled, and then either disentangling it or not. So one could think about, in the above scenario, whether A could tell B ahead of time which Bell measurements have been successful (and perhaps in which way), and then, when the time comes to send a message, to disentangle them or not. Here I don't know what options are available for doing so, however this sounds like one of the more promising ideas. Of course, there seems to be some underlying principle that makes all such attempts run into some kind of difficulty. But as long as that principle isn't clearly formulated other than through pre-quantum relativity theory, one might as well keep trying... :) ...and, who knows, perhaps there are other features of our universe yet undiscovered. After all, even Einstein was convinced that there would be no 'real' non-locality and/or non-realism at all, not even the kind that has been proven now, even though he was one of those who discovered that it was implied in quantum theory.

    As far as I understand, it was discovered only as recently as in 2001, that the influence of random values in entanglement can be alleviated such that entanglement becomes usable for building quantum computers. Personally, my intuition is that there should be a way, given that there is entanglement at all, even though the current state of art in entanglement gives little hope for a FTL communication that doesn't require an additional classical communication.
     
  9. Jan 3, 2008 #8
    Personally, my entirely non-scientific hunch is that if FTL communication is impossible with entanglement, it's because there is a yet-undiscovered local hidden variable model that can explain the correlations. All these other reasons that FTL scenarios don't work seem too damned convenient - I prefer not to believe that the universe is conspiring against us. :)
     
  10. Jan 3, 2008 #9
    Ok, let us know when you find this local hidden variable model. Meanwhile entanglement is being used to build quantum computers, and whether FTL or not, my non-scientific reasoning is that if entanglement didn't provide anything other than random correlations, then there wouldn't be any point in using it in quantum computers. Or maybe there isn't? :)

    [Edit added:] Actually, I should say: there wouldn't be much of a point... :)
     
    Last edited: Jan 3, 2008
  11. Jan 3, 2008 #10
    Haha. I don't expect to find it. :) Rather I expect FTL communication will happen, mainly because I think our linear notions of causality (which, at present, suggest FTL communication leads to unsolveable paradoxes) are incomplete.

    [Edit]There's one local, deterministic hidden variable theory that could explain this whole business. The wavefunction of the entire universe has been preordained from the get-go, and the polarization settings of every experiment are reflected in this wavefunction even before the photons are launched. In other words, no free-will.
     
    Last edited: Jan 3, 2008
  12. Jan 3, 2008 #11
    Oh, you do expect it will happen. Ok. :)

    The latter is what A.Zeilinger calls "superdeterminism". He argues you could drive the measurement angles from signals coming from opposite directions of outer space, and that it would be odd if these signals were synchronized with the correlations in the photons.

    My proposal would be to drive the angles from temperature measurements at opposite sides of the planet, on one side measured in Celsius, on the other in Fahrenheit. And then drive the angles based on whether the third decimal is an odd or an even number. Now how does the universe make sure from the get-go that all this is still correlated with the photons states in the experiment? :)
     
  13. Jan 3, 2008 #12
    Well, superdeterminism seems an all-or-nothing proposition. Whatever force caused everything to be the way it is would be, by definition, omnipotent, so whether the signals came from the lab, opposite ends of the earth, or other galaxies, is just a question of scale and therefore wouldn't matter.

    The only thing I don't believe is that the universe is conspiring to give us "hints" of FTL without ever succeeding. It seems way too "cute." Either we're wrong, and there's no information being exchanged, or there is information exchanged, and we'll find a way to identify and utilize it.
     
  14. Jan 3, 2008 #13
    Superdeterminism: Yes, but my point is that in addition to being omnipotent, it would also have to very, what you call "cute", yet even more so. :)

    FTL: It seems we are going in the latter direction, already past the hints, yet still quite slowly. On the other hand, a lot seems to have happened since 1998.
     
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